I didn't see any FRU (field replaceable unit) part numbers but drive mounting is usually something you can make yourself. In a pinch I've used tie wraps or zip ties to hold a drive in place.
I'll soon buy a used HP Elite 8000 desktop convertible minitower which has no memory nor HDD (and perhaps no caddy for the HDD). If I need a caddy for the HDD, is this a cheap, plastic item that a 3.5" HDD slips into? Where are these sold? Do some of these Elite 8000 desktop convertible minitowers not use a caddy but just screws to hold in the HDD? Where are the screws obtained?